Our past and future experiments are directed at: 1. Understanding the organization, expression and regulation of SV40's early genes and how their gene products function in the initiation and maintenance of the oncogenic phenotype. 2. Developing and using SV40 DNA vectors to introduce new genes into the genome of cultured cells to provide experimental models for studying the mechanisms of gene expression and regulation in mammalian cells. Our more immediate goals towards these objectives are: 1. To construct appropriate recombinant viral genomes which can be maintained in either an integrated or autonomously replicating state in cultured animal and human cells. 2. To use such recombinant DNAs to explore the function of viral regulatory elements, particularly those that control the expression of the oncogenetic functions. 3. To analyze the patterns and mechanisms of homologous and non-homologous recombination in mammalian cells.